This application relates generally to information handling systems and more particularly to a magnetic circuit of a voice coil motor in an information handling system.
One function of a disc drive is reliable storage and retrieval of information. Using one common implementation of a disc drive as an example, data is stored on one or more discs coated with a magnetizable medium. Data is written to the discs by an array of transducers, typically referred to as read/write transducers, mounted to an actuator assembly for movement of the transducers relative to the discs. The information is stored on a plurality of concentric circular tracks on the discs until such time that the data is read from the discs by the read/write transducers. Each of the concentric tracks is typically divided into a plurality of separately addressable data sectors. The transducers are used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the transducer senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the transducer over the center of the desired track.
Conventionally, the transducers are positioned with respect to the disc surfaces by one or more actuator arms controlled through a voice coil motor. The voice coil motor is responsible for pivoting the actuator arms about a pivot shaft, thus moving the transducers across the disc surfaces. The actuator arm thus allows the transducers to move back and forth in an arcuate fashion between an inner radius and an outer radius of the discs. The actuator arm is driven by a control signal fed to a voice coil motor coil coupled to the rear end of the actuator arm.
The coil is immersed in the magnetic field of a magnetic circuit of the voice coil motor. With respect to conventional voice coil motor implementations, the magnetic circuit comprises one or more permanent magnet pairs adjacent to magnetically permeable magnet plates. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit such that the coil, as well as the transducer(s), experience direct rotational forces or torques about an axis of a rotatable assembly. Such rotational forces selectively position the transducer over the desired new track or maintain the position of the transducer over the desired current track. A conventional implementation of the magnetic circuit of the voice coil motor is shown in FIG. 3. A second conventional implementation of the magnetic circuit of the voice coil motor is shown in FIG. 4.
A servo control system is used to sense the position of the actuator arm and control the movement of the transducer above the disc using servo signals read from the servo segments on the disc surface in the disc drive. The servo control system relies on servo information stored on the disc. The signals from this information generally indicate the present position of the transducer with respect to the disc, i.e., the current track position. The servo control system uses the sensed information to maintain transducer position or determine how to optimally move the transducer to a new position centered above a desired track. The servo system then delivers a control signal to the coil of the voice coil motor to rotate the actuator arm to position the transducer over a desired new track or maintain the position over the desired current track.
As shown in FIG. 3, in a typical voice coil motor 324 employing two parallel magnet pairs 342 and 344 coupled to an upper magnetically permeable plate 343 and a lower magnetically permeable plate 345, respectively, the lines of magnetic flux 346 generated by the permanent magnet pairs 342 and 344 tend to cross an air gap 348 located between an upper surface 350 of the lower magnet pair 342 and a lower surface 352 of the upper magnet pair 344 in a generally orthogonal direction to surfaces 350 and 352 of the permanent magnet pairs 342 and 344. When these xe2x80x9corthogonalxe2x80x9d lines of magnetic flux 346 interact with the flux generated by a coil 326, the resultant torque induced in the VCM 324 is primarily of the direct type, as described above. Put another way, when the flux generated by the parallel magnet pairs 342 and 344 of the VCM 324, interacts with the flux generated by current in the coil 326, balanced forces or torques act upon the VCM 324.
The orthogonal orientation of the flux lines 346 relative to the surfaces 350 and 352 of the permanent magnet pairs 342 and 344 is thought to be due to a xe2x80x9csteeringxe2x80x9d effect the oppositely facing north and south facing magnetic poles 362 and 364 of the permanent magnet pairs 342 and 344 have on the magnetic flux 346. That is, the oppositely facing north and south facing magnetic poles 362 and 364 of the permanent magnet pairs 342 and 344 tend to guide the lines of magnetic flux 346 across the air gap 348 located between the permanent magnet pairs 342 and 344 in a generally orthogonal direction to the surfaces 350 and 352 of the permanent magnet pairs 342 and 344.
In contrast, as shown in FIG. 4, it has been observed that without the guiding influence of the oppositely facing south and north magnetic poles, lines of magnetic flux 446 generated in a VCM 424 having a single magnet pair 444 tend to xe2x80x9cfringexe2x80x9d as they cross the air gap 448 between the permanent magnet pair 444 and the upper magnetically permeable plate 440. That is, the lines of magnetic flux 446 generated in the VCM 424 employing a single magnet pair 444 do not typically remain orthogonal to the upper surface 450 of the permanent magnet pair 444. It is believed that when these xe2x80x9cnon-orthogonalxe2x80x9d flux lines interact with the flux generated by the coil 426, the result is unbalanced forces and moments acting on the coil 426. Such unbalanced forces and moments typically lead to an undesirable increase in pitch torque and roll torque in the VCM 424.
Although the conventional implementation shown in FIG. 3 is desirable to somewhat alleviate unbalanced forces and moments associated with non-orthogonal flux lines, the parallel magnet design in FIG. 3 is associated with relatively greater manufacturing costs than the design shown in FIG. 4. In contrast, even though the magnetic circuit implementation shown in FIG. 4 is relatively inexpensive to manufacture when compared to the conventional implementation shown FIG. 3, the design shown in FIG. 4 is associated with potentially unbalanced forces and moments.
Against this backdrop the present invention has been developed. The present invention relates to a magnetic circuit of a voice coil motor incorporating a single permanent magnetic portion extending from a first plate and a raised plate portion protruding toward the first plate from a second plate. The raised plate portion protrudes from the second plate to interact with the magnetic portion extending from the first plate in order to reduce the occurrence of non-orthogonal flux lines in the voice coil motor. Accordingly, the voice coil motor employs a single magnetic portion and an opposite pole piece shaped to generate orthogonal, as opposed to non-orthogonal, flux lines relative to the surface of the permanent magnetic portion to ensure relatively balanced forces and moments acting upon the coil of the voice coil motor.
In accordance with one embodiment, a disc drive includes a voice coil motor for positioning a transducer over a data disc surface of a data storage disc rotatably mounted on a base plate. An actuator, which is coupled to the voice coil motor, is mounted on the base plate adjacent the disc for moving the transducer over the disc surface. The voice coil motor includes a voice coil, a first plate, a permanent magnet pair coupled to the first plate, a second plate having a planar surface and a raised plate portion protruding from the second plate planar surface toward the permanent magnet pair. The permanent magnet pair defines a magnetic planar surface and produces a magnetic flux. The second plate is positioned in spaced relation to the first plate thereby forming an air gap between the raised plate portion and the permanent magnet pair through which the voice coil is free to move. The raised plate portion defines a raised planar surface opposite the magnetic planar surface such that the magnetic flux passing across the air gap between the permanent magnet pair and the raised plate portion is substantially uniformly directed orthogonal to the magnetic planar surface.
These and various other features, as well as advantages which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.